Glaucoma is a progressive optic neuropathy, which induces irreversible structural damage of the retina that is manifested, for example, in retinal ganglion cell loss and thinning, due to axon loss, of the retinal nerve fiber layer (RNFL).1,2 Glaucomatous damage occurs along axon bundle and creates a localized, wedge-shaped RNFL defect, evident in a red-free fundus photograph (FIG. 1) because axons are originated from the retinal ganglion cells and converge at the optical nerve head (ONH).1,2 The cross-sectional tissue structure of the retina can be revealed by using optical coherence tomography (OCT), which is a non-contact and non-invasive imaging modality.3 The conventional time domain-OCT, 3.4-mm diameter circle scan, centered on the ONH (FIG. 2), enables quantitative RNFL analysis.4,5 Furthermore, multiple repeated RNFL thickness measurements, made using TD-OCT circle scans in different time points, also provide a useful approach to detecting glaucoma progression, earlier than visual field (VF) reduction.6 
One limitation of TD-OCT circle scan is measurement variability in long term follow-up because operator manually places the TD-OCT circle scan around the ONH (FIG. 3).7,8 There is a method, called “Repeat-scan function,” implemented via Stratus OCT system software (CZMI). With this function, an operator is able to place the TD-OCT circle scan with a unique landmark at the base-line (first visit) scan, in order that the same scan at the second visit can be performed on, the same location. The improvement thus achieved is small, i.e., there is significant improvement but only in temporal quadrant, due to the difficulty of registration.9 
Spectral domain OCT (SD-OCT) technology, with faster scanning speed and higher axial resolution relative to the conventional TD-OCT, has allowed three-dimensional (3D) volume sampling by raster scanning in the region of interest.10-13 An SD-OCT fundus image, which can be obtained by summing the back scattered signal at each transverse point of a retinal raster scan, visualizes especially the presence of horizontal eye motion during SD-OCT scanning, because the discontinuities of retinal blood vessels appearing on SD-OCT fundus image are generated by saccadic eye motion.11-13 3D SD-OCT volume data also enable a virtual OCT cross-sectional (B-scan) image along any sampling line (curved or straight). For example, TD-OCT circle scan can be virtually performed within 3D SD-OCT volume (FIG. 4D, circle). The virtually sampled B-scan after near perfect registration can be used for comparison between TD-OCT and SD-OCT.14 In addition, a two-dimensional (2D) RNFL thickness map (FIG. 4C) can be created by segmenting RNFL on each B-scan of 3D SD-OCT volume scan.15 